Process for the catalytic oxidation of hydrocarbons and other compounds utilizing ceramic foam supported catalysts. Included are processes for the preparation of carbon monoxide and hydrogen (certain mixtures of which are also known as synthesis gas or syngas) by catalytic partial oxidation (CPO) of low carbon number hydrocarbon feed streams, such as methane; catalytic oxidation of ethylene; catalytic oxidation of ammonia, etc. Syngas is useful for the preparation of a variety of other valuable chemical compounds, such as by application of the Fischer-Tropsch process; ethylene oxide and nitric acid also have significant commercially utility. Additionally, catalytic oxidation can be applied to processes for the reaction of hydrocarbon fuel with air and steam under conditions suitable for use in fuel cell reformer applications.
1. Background of the Invention
The combustion of methane gas at elevated temperature, e.g., 1000xc2x0 F. (538xc2x0 C.) is highly exothermic and produces CO2 and H2O according to the following stoichiometry:
CH4+2O2xe2x86x92CO2+2H2Oxe2x80x83xe2x80x83(xe2x88x92190.3 kcal/g mol CH4)
The gases formed in such a reaction are not directly useful for the production of valuable chemical compounds. Furthermore, if efforts were made to produce valuable products from water and carbon dioxide, the high temperatures generated would present problems with respect to reactors, catalysts and other process equipment.
Conversely, it is known to produce a chemically useful mixture of CO and H2 gases, also known as synthesis gas or syngas, from methane and other light hydrocarbon gases, by various reactions, including partial oxidation, steam- or CO2-reforming, or a combination of these chemistries. The partial oxidation reaction of methane is a less highly exothermic reaction which, depending upon the relative proportions of the methane and oxygen and the reaction conditions, can proceed according to the following reaction paths: 
It is most desirable to enable the partial oxidation reaction to proceed according to the last reaction scheme. This results in certain advantages, including: (1) producing the most valuable syngas mixture; (2) minimizing the amount of heat produced (thereby protecting the apparatus and the catalyst bed); and (3) reducing the formation of steam (thereby increasing the yield of hydrogen and carbon monoxide). Any incidentally produced, or added, steam can be further converted by the steam-reforming reaction into additional useful syngas components.
Fuel cells produce electricity by converting reactants such as hydrogen and oxygen into products such as water. One method of providing hydrogen for use in a fuel cell is to oxidize a liquid hydrocarbon fuel in a fuel cell reformer that is a component of the fuel cell system. Improving the conversion of liquid hydrocarbons to hydrogen for use as a fuel in fuel cell processes is part of the continuing effort to commercialize fuel cell technology.
Ammonia oxidation is used to produce nitric acid by oxidizing ammonia in the presence of the catalyst, typically a pad of platinum-containing gauzes, to produce hot nitric oxide. The nitric oxide is then quenched and oxidized, e.g., in the presence of air, to form nitrogen (IV) oxides, which then react with water to form nitric acid. Uneven flow distribution of the ammonia through the gauze pad (i.e., the catalyst bed) can result in an undesirable reaction that produces nitrogen and water; improvements in catalyst technology could improve such a commercially important process.
The oxidation of ethylene to ethylene oxide is typically carried out in the presence of a silver-containing catalyst supported on a low surface area ( less than 1 m2/g) alpha-alumina carrier or substrate. It has been suggested that such a substrate provides large pores that are effective in avoiding a diffusion limited reaction regime having reduced selectivity (C. N. Satterfield, xe2x80x9cHeterogeneous Catalysis in Industrial Practicexe2x80x9d , 282, 2d Ed.,1991, McGraw-Hill). Improved catalyst technology, including improvements to the carrier or substrate per se, may thus lead to improvements in this commercially valuable process.
Copending patent application, U.S. Ser. No. 08/484378, filed Jan. 14, 2000, describes a multistage CPO process wherein control of specific process conditions, including preheat of the gaseous feedstreams and control of interstage temperature and the number of stages, leads to improved methane conversion and syngas product (hydrogen and CO) selectivity (the text of this application is incorporated herein for all permitted purposes).
A catalytic partial oxidation process utilizing a ceramic foam catalyst support is described by Vonkeman et al. in EP 576 096 B1 (Shell). The support is disclosed as having a high xe2x80x9ctortuosityxe2x80x9d, which is defined as the ratio of the length of the path followed by a gas flowing through a bed of the catalyst to the length of the shortest straight line path through the catalyst bed. The general range of conditions disclosed for conducting a CPO process with such a catalyst includes an oxygen to carbon molar ratio in the range of from 0.45 to 0.75, elevated pressure (up to 100 bar), space velocity of from 20,000 to 25,000,000 Nl/l/hr, hydrocarbon and oxygen-containing gas, feed preheat, and a reaction zone necessarily under adiabatic conditions. The catalyst was prepared by impregnating a commercially available, particulate alpha-alumina carrier with an aqueous solution of chloroplatinic acid, followed by drying and calcining in order to deposit platinum.
Kumar et al., WO 96/16737 (Shell) also describe a specific process for the preparation of a high tortuosity ceramic foam support impregnated with high loadings of an inorganic oxide. A catalyst containing a catalytically active component is prepared with the support for use in a hydrocarbon feedstock oxidation process.
Van Grinsven, et al., EP 537 862 A1 (Shell), describe a CPO process using a noble metal catalyst supported on an alpha-alumina support having xe2x80x9cvery large poresxe2x80x9d, defined as requiring that at least 90% of the pore volume are pores greater than 2,000 nm (equivalent to 20,000 angstroms or 2 microns). Optionally the supported catalyst can be prepared using a co-impregnated salt of at least one metal which forms, upon calcination, an oxide that cannot easily be reduced (Al2O3 being most preferred).
Jacobs et al., U.S. Pat. No. 5,510,056 (Shell) disclose that successful operation of a CPO process on a commercial scale requires high conversion of the hydrocarbon feedstock at high space velocities, using mixtures of an oxygen-containing gas and methane in a preferred O2 to carbon atom ratio (in the region of the stoichiometric ratio of about 1:2, or 0.5), which mixtures are preferably preheated and are at elevated pressures. The advance described in Jacobs also requires the use of a high tortuosity, high porosity catalyst carrier.
It is disclosed in EP 303 438 (assigned to Davy McKee Corp.), to conduct a CPO process using previously formed mixtures of high temperature, high pressure methane and oxygen gases and, optionally, steam at space velocities up to 500,000 hrxe2x88x921, using a mixing and distributing means in order to thoroughly premix the gases prior to introduction to the catalyst. It is the objective in this disclosure to operate the CPO process in a mass-transfer-controlled regime and to introduce the gas mixture at or above its autoignition temperature.
EP 842 894 A1 discloses a process and apparatus for catalytic partial oxidation of a hydrocarbon wherein the use of several stages is proposed. The reference states that in each stage there is used xe2x80x9ca small fraction of the stoichiometric amount of oxygen required for the reactionxe2x80x9d to prevent the generation of high temperatures in the reactor as a consequence of xe2x80x9cexcessivexe2x80x9d concentrations of oxygen. Furthermore, it is disclosed that the hydrocarbon feed is mixed with oxygen and preheated to a temperature in the range of 300-400xc2x0 C. and the reaction is performed at substantially the same temperature in all stages by cooling the reaction mixture in each stage.
GB Patent Application 2311790 discloses a two stage process and the use of a specifically defined catalyst whereby in a second stage a second synthesis gas is produced utilizing a first synthesis gas as feed gas, in combination with oxygen to cause partial oxidation of unreacted methane.
Staged oxygen addition has been disclosed as providing possible improvements to some of the difficulties encountered in CPO processes. xe2x80x9cCO2 Reforming and Partial Oxidation of Methane,xe2x80x9d Topics in Catalysis 3 (1996) 299-311, recommends a staged addition of O2 to the reactor during methane oxidation in a two stage process, including total methane oxidation followed by reforming in the presence of the formed CO2 and H2O. Oxygen staging is said to lead to a flattening of the temperature profile along the reactor. However, it is also stated that xe2x80x9clowering the O2/CH4 ratio will make carbon deposition thermodynamically more favourable and thus lead to deactivation of the catalyst.xe2x80x9d (Id., p.308). The experimental results reported in this reference are expressed as a function of the catalyst-bed exit temperature rather than feed temperature.
The oxidation of ammonia using a reticulated ceramic foam coated with cobalt and zinc compounds is disclosed by L. E. Campbell in U.S. Pat. No. 5,336,656 (Scientific Design Company), as well as a process for preparing the catalyst. No special characteristics are attributed to the ceramic foam structure and it is merely described as commercially available and having between 10 to 100 pores per linear inch (ppi).
Several patents assigned to Dytech Corporation Limited disclose foamed ceramic catalyst supports prepared by a process which leads to articles allegedly having pores xe2x80x9cremarkably uniformxe2x80x9d in size (see, e.g., U.S. Pat. No. 5,563,106, at col. 4, line 42, J. G. P. Binner et al.). The reference also suggests that such porous articles can be used as catalyst supports. The ceramic foam support is prepared by introducing gas bubbles into a dispersion of fine inorganic oxide particles, optionally also including a binder, drying the wet foamed article and then sintering to obtain the final article. The reference acknowledges that the properties of the foamed ceramic can be varied widely by varying the process conditions, including the viscosity of the dispersion, the speed of stirring when introducing gas bubbles, the presence or absence of a binder and the nature of the binder, and the conditions of drying. Furthermore, it is also disclosed that articles may be produced having pores that may be open or closed. Production of foamed ceramic articles by various methods is also described by R. M. Sambrook et al. in U.S. Pat. Nos. 5,705,448; 5,772,953; and 5,922,272 (all assigned to Dytech Corporation Limited). In particular, it is suggested in the xe2x80x2448 patent that an interconnected open pore structure may be desired for certain applications.
Various technical articles have been published or presented relating to the preparation of ceramic foam products by various methods. The products are typically characterized by the number of pores per inch (ppi) as well as strength, thermal and density characteristics (see, e.g., xe2x80x9cPreparation and properties of ceramic foam catalyst supportsxe2x80x9d, M. V. Twigg et al., presented at the Sixth International Symposium on the Scientific Bases for the Preparation of Heterogeneous Catalysts, (Sep. 5-8, 1994); xe2x80x9cPorous Ceramicsxe2x80x9d, American Ceramic Society 71(1992)1674-1682).
While the above references disclose certain aspects of hydrocarbon oxidation based on CPO and staged oxygen addition, there still exists a need for improvements in processes for the oxidation of hydrocarbons and other compounds, particularly related to improvements in yield and selectivity.
A process for the catalytic oxidation of a feed comprising contacting said feed with an oxygen-containing gas in the presence of a catalyst in a reaction zone, wherein:
(a) said catalyst comprises at least one catalytically active metal selected from the group consisting of silver and the elements of Group VIII of the Periodic Table of the Elements supported on a porous ceramic carrier;
(b) said catalyst is retained within said reaction zone in a fixed arrangement; and
(c) said carrier has a distribution of total pores wherein:
(1) about 70 percent of said total pores have a volume to surface area ratio (V/S) that is within about 20 percent of the mean V/S value for said total pores and no pores have a volume to surface area ratio that is greater than twice the mean V/S value for said total pores; and
(2) for about 70 percent of said total pores, the pore-to-pore distance between neighboring pores is within about 25 percent of the mean pore-to-pore distance between neighboring pores; and
(3) about 50 percent of said total pores have a coordination number that is within about 50 percent of the mean coordination number between neighboring pores and wherein less than about 15 percent of said total pores have a coordination number that is less than 3. Furthermore, in a preferred carrier,
(4) about 70 percent of the total pores have a pore throat area that is within about 50 percent of the mean pore throat area for the total pores; preferably, about 70 percent of the total pores have a pore throat area that is within about 30 percent of the mean pore throat area for the total pores.
Preferably, the oxidation process comprises a co-fed, single stage catalytic partial oxidation (CPO) process or a multistage, staged oxygen catalytic partial oxidation process of less than or equal to about five stages and including a first stage preheat temperature of greater than about 550xc2x0 C. and wherein the temperature of the product mixture in each stage following the first stage is at least about 700xc2x0 C. Alternatively, the oxidation process comprises a fuel cell reformer wherein CPO and optionally, steam reforming and water-gas shift reactions are carried out using a liquid hydrocarbon fuel in order to generate hydrogen for use as the fuel in a fuel cell.